1. Field of the Invention
This invention relates to a multi-channel type optical transmitter (and an optical transmission method) which monitors an optical output, and control multiple laser diodes (LD) based on the monitored optical output.
2. Description of the Related Art
When data are transmitted by an optical communication in parallel, a laser array device which integrates a plurality of LDs is used as a light source. As the laser array device, for example, a semiconductor laser array device is known, in which one of multiple semiconductor laser units is set as a reference semiconductor laser unit. Feedback control is performed by monitoring light only about the reference semiconductor laser unit. The other semiconductor laser units are controlled to keep a ratio between a bias current of the reference semiconductor laser unit and a bias current of the other semiconductor laser units constant. An optical output of each semiconductor laser unit is stabilized (e.g., see Japanese Patent JP-B-3130571, paragraph 0008˜0012, FIG. 1).
However, according to the conventional structures, since the optical outputs of the multiple semiconductor laser units are not detected individually, when there is a variation of a thermal characteristic between the multiple semiconductor laser units, it is difficult to control exactly to keep the optical outputs constant. Further, since the optical outputs of the multiple semiconductor laser units are not detected individually, it is impossible to detect a performance degradation or a failure of each semiconductor laser unit over their service life.
Therefore, prior to the present invention, there has been no conventional system (or method) which can provide an optical transmitter (or an optical transmission method) which can monitor individually each of the optical outputs of the semiconductor laser units, and control individually each light-emitting element of the semiconductor laser units according to the monitored result.